The parents sometimes arrive with kids who are 12, 13, 14 and they use the schools for a babysitting service while they work in the fields. This is especially true for kids who speak only their native tongue which is not Spanish but rather Mixteco.
It's simple to track; one just goes to the school website and looks for Mixteco-speakers in the 8th grade and 9th grade and there are quite a few but in the 10th grade, there are no Mixteco speakers. They did not learn to speak English. They dropped out and went to work picking strawberries.